Aircraft manufacturers continually strive for ways to increase aircraft performance and reduce manufacturing costs. One well-known method for increasing aircraft performance is to reduce airframe weight through the use of composite materials having relatively high strength-to-weight ratios. Composite materials have been used on airframes for fighter aircraft, high-performance private aircraft, and business jets. Larger aircraft, however, such as large commercial transport aircraft, typically use metallic materials for all or most of the structure. The fuselage shells for commercial transport aircraft, for example, are typically manufactured from aluminum and other metals.
Conventional methods for manufacturing business jet airframes with composite materials typically require extensive tooling fixtures and labor-intensive assembly procedures. One known method used by the Raytheon Aircraft Company of Wichita, Kans., to manufacture the Premier I and Hawker Horizon business jets involves wrapping carbon fibers around a rotating mandrel with an automated fiber placement system. The mandrel provides the basic shape of a fuselage section. The carbon fibers are preimpregnated with a thermoset epoxy resin, and they are applied over the rotating mandrel in multiple plies to form an interior skin of the fuselage section. The interior skin is then covered with a layer of honeycomb core. The fiber placement system then applies additional plies of preimpregnated carbon fibers over the honeycomb core to form an exterior skin that results in a sandwich structure. The final ply includes a hybrid fabric of carbon fiber and fine metallic wires to provide lightning strike protection.
The Premier I fuselage includes two composite fuselage sections formed in the foregoing manner. The Hawker Horizon fuselage includes three sections formed in this manner. After forming, the respective fuselage sections are bonded together along circumferential joints to form the complete fuselage shell. Another method for forming composite fuselage shells in accordance with the prior art involves forming fuselage halves or quarter-panels separately (for example, by a fabric lay-up process), and then joining the separate parts together along longitudinal joints to form a complete fuselage cross-section.
Filament winding, fiber placement, and tape laying are three known methods for applying unidirectional composite fibers to a rotating mandrel to form a continuous cylindrical skin. In a filament winding process, the mandrel is typically suspended horizontally between end supports. The mandrel rotates about the horizontal axis as a fiber application instrument moves back and forth along the length of the mandrel, placing fiber onto the mandrel in a predetermined configuration. In most applications, the filament winding apparatus passes the fiber material through a resin “bath” just before the material touches the mandrel. This is called “wet winding.” In other applications, the fiber has been preimpregnated with resin, eliminating the need for the resin bath. Following oven or autoclave curing of the resin, the mandrel can remain in place and become part of the wound component, or it can be removed.
The fiber placement process typically involves the automated placement of multiple “tows” (i.e., untwisted bundles of continuous filaments, such as carbon or graphite fibers, preimpregnated with a thermoset resin material such as epoxy) tape, or slit tape onto a rotating mandrel at high speed. A typical tow is between about 0.12″ and 0.25″ wide when flattened. Conventional fiber placement machines dispense multiple tows to a movable payoff head that collimates the tows (i.e., renders the tows parallel) and applies the tows to the rotating mandrel surface using one or more compaction rollers that compress the tows against the surface. In addition, such machines typically include means for dispensing, clamping, cutting and restarting individual tows during placement.
Tape laying is similar to the fiber placement process described above except that preimpregnated fiber tape, rather than individual tows, is laid down on the rotating mandrel to form the part. One form of tape includes a paper backing that maintains the width and orientation of the fibers. The paper backing is removed during application. Slit tape is tape that has been slit after being produced in standard widths by the manufacturer. Slitting the tape results in narrower widths that allow enhanced stearability and tailoring during application to achieve producibility and design objectives. Slit tape can have widths varying from about 0.12 inch up to about 6 inches, and may or may not include backing paper. Another form of tape includes multiple individual fibers woven together with a cloth material. As used throughout this disclosure, unless otherwise indicated, the term “tape” refers to tape, tape with backing paper, slit tape, and other types of composite material in tape form for use in manufacturing composite structures. Tape laying is often used for parts with highly complex contours or angles because the tape allows relatively easy directional changes.